Methamidophos resistance and acetylcholinesterase (AChE) insensitivity to methamidophos, dichlorvos, and carbofuran were determined in the field populations of Plutella xylostella (L.) (Lepidoptera: Yponomeutidae) and its parasitoid Cotesia plutellae Kurdjumov (Hymenoptera: Braconidae) collected from the corresponding hosts between October 1998 and December 2003 in Fuzhou and Minhou, Fijian, China. Resistance levels to methamidophos and AChE insensitivity to the three insecticides in the two species of insects were high during autumn and spring and low during summer. Resistance to methamidophos was 15.3- and 12.6-fold higher in resistant F₀ parents of P. xylostella and C. plutellae than in their susceptible F₁₁ progeny, respectively. The bimolecular rate constant (k_i) values of AChE to methamidophos, dichlorvos, and carbofuran were 4.6-, 6.3-, and 7.7-fold higher in F₁₁ progeny of P. xylostella, and 3.7-, 4.5-, and 3.7-fold higher in F₁₁ progeny of C. plutellae than those in their F₀ parents, respectively. Compared with susceptible F₁₁ progeny, the resistance ratios for methamidophos were 4.2–29.8 and 3.8–13.1 in 21 field populations of P. xylostella and C. plutellae, respectively. The k_i values of AChE to methamidophos, dichlorvos, and carbofuran were 2.0–21.6-, 3.6–9.5-, and 2.6–9.2-fold higher in F₁₁ progeny of P. xylostella, and 1.8–7.6-, 1.9–4.6-, and 2.2–7.6-fold higher in F₁₁ progeny of C. plutellae than those in 21 field populations, respectively. Significant correlative variations of methamidophos resistance as well as significant correlative variations of k_i values of AChE to insecticides between the two species of insects also were found in space and time. The k_i values of AChE to insecticides in C. plutellae were far higher than those in P. xylostella. There were no obvious differences in the K_m and V_max of AChE between F₀ parents and F₁₁ progeny of P. xylostella and C. plutellae, respectively. But carboxylesterase activity was 1.6-fold higher in F₀ parents of C. plutellae than in F₁₁ progeny, and glutathione S-transferase activity was 1.5-fold higher in F₀ parents of P. xylostella than in F₁₁ progeny. The results suggested that the AChE insensitivity to insecticides might play the most important role in methamidophos resistance in the two species of insects. From these results, a spatial and temporal correlative evolution of methamidophos resistance and insensitive AChE was found to exist between P. xylostella and C. plutellae.